1. Field of the Invention
The present invention relates to the maintenance of uniform optical properties in transmissive and reflective devices and, more particularly, to maintaining such properties for establishing a predetermined thermal pattern throughout such devices.
2. Description of Related Art and Other Considerations
In order to operate properly, such optical devices as windows and reflectors must be free from substantial distortion, which may be caused by non-uniform or other undesired thermal distributions in the optical device, or by the formation of ice thereon.
Such icing may occur from flight through an icing cloud or on the ground or from rapid thermal transients in-flight. A cover of ice over the window will blind the optical viewing, such as in a forward-looking infrared (FLIR) system, while ice on a viewing turret in which the window, e.g., of germanium, is mounted, can restrict the full range of motion. Protection against these conditions is provided by resistive heating of the germanium window substrate and a heater strip around the azimuth gimbal/base interface of the turret. This design provides for full panoramic viewing with the FLIR.
Historically, anti-icing has been implemented and/or studied for many systems. In one sight a conductive coating was used to heat the visible window. In another, two different heated windows were employed, one using a metallized coating and the other a hot air system similar to a thermopane.
A further system utilized a large rectangular window made of polycrystalline germanium. That window was electrically heated using the bulk resistance of the semiconductor material. However, two major problems with resistively heating polycrystalline germanium are accelerated corrosion by preferential etching and non-uniform heating. When the anti-reflection coating on the substrate contains pin holes, moisture together with dissolved carbon dioxide and oxygen can penetrate to the substrate. If pin holes are near grain boundaries, corrosion will result when voltage is applied to the window. Corrosion is greatly accelerated in the presence of 2 micron radiation and an ultimate failure of the window will occur by crazing. Accordingly, such windows must be regularly inspected, and removed from service as required. Those windows can then be repolished, recoated, and returned to service. Alternatively, because these problems are caused by grain boundaries in the polycrystalline material, they can be avoided by using single crystal material.
Based upon this and other experience, various requirements are placed upon the design of the window, such as by environmental specifications, existing hardware, and the end use for which the window and viewing system are intended, as defined by a particular program.
Icing specifications take two different forms: climatic and equipment. Climatic requirements describe the meterological conditions under which icing may occur and the extent of the condition. For example, icing generally occurs at altitudes from sea level to 22,000 feet and ambient temperatures from -4.degree. F. to 32.degree. F. A "moderate" condition is one in which 1/2 inch of ice can accumulate on a small probe in 20 miles.
Hardware specifications expand upon the environmental conditions and also require specific design criteria. Thus, for many end-use requirements of the window, only thermal anti-icing may be considered and heated surfaces shall be running wet with a temperature above 35.degree. F. in a 0.degree. F. environment. In addition, the aircraft speed shall be at a maximum. Using the various specifications, a heating requirement of 3.9 watts/sq. in, for an examplary design, may be arrived at.
Other requirements are established by the program. Aircraft subjected to the less severe "trace" icing condition need not be protected. Also, an icing condition and the anti-icing equipment to mitigate that condition can have an impact on performance. It is therefore reasonable to require that the anti-ice system have no influence on performance during non-operational periods. When operating, the anti-icing equipment shall only cause "minor" degradation to system performance.
Regarding ground icing, the sight generally is cleared in the same manner as in the rest of the aircraft.
For a particular turret, in designing a minimum drag turret with a full azimuth field of regard, a spherical turret was faired into a cylindrical base section. To maintain the lowest aerodynamic turret torques, it was necessary to minimize the area of all flat surfaces. Thus, except for a spherical window, which has optical power, a circular window sized as close as possible to the extent of the incoming ray bundle is desirable. Since the concern was with a low speed aircraft, aerodynamic heating was not a significant consideration, and germanium was chosen for the window material as having superior optical performance.
In an anti-iced design, it is necessary to consider optical degradation both during periods when the de-icing implementation is and is not energized. A conductive edge heater induces large radial temperature gradients into a window, and the gradients affect the quality of the imagery. This is particularly true with germanium due to its relatively large change of index of refraction with temperature (dn/dT). Combined with the coefficient of thermal expansion of the material, the gradients will yield a variable but weak lens. Since the window is tilted to control narcissus, this "weak lens" will introduce astigmatism into the system.
Another approach to an electrically heated window utilizes surface heating by a conductive coating or deposited (or embedded) elements. Such a window, whether through obscuration or transmission losses, will lower the incoming signal. In addition, any hot spots caused by the heater elements will introduce noise (albeit out of focus) into the system.
A third alternative employs indirect electrical heating with a thermopane. In such a system hot air is passed between two window panes and heats the outer pane by convection. Such a scheme is usable on a visible (or near visible) system using a window of glass due to the negligible change of index of refraction with temperature and low coefficient of thermal expansion. Such a configuration will have significant temperature gradients in the direction of flow and non-uniform gradients perpendicular due to the circular shape. For glass windows, these gradients do not pose a problem. However, in windows of such a semiconductive material as germanium, these gradients, when combined, with the large dn/dT of the semiconductor material, will degrade the imagery.
A window fabricated from germanium is both electrically and thermally conductive. Thus, anti-icing has been provided directly by resistive heating of the bulk material. Such a scheme will have no impact on the system performance when not energized and is chosen to minimize degradation during operation.
Germanium, a semiconductor, can be electrically heated by simply passing a current through it. However, its resistivity can decrease dramatically as temperature increases due to thermal excitation of free carriers which results in an increase in the infrared optical absorption. Thus, a thermal control system must be provided to prevent thermal runaway. The index of refraction is also highly temperature dependent. Therefore, uniform heating is required to minimize optical degradation.
Normally, resistively heated germanium windows are of a rectangular design and constant thickness so that the heating essentially is uniform. A typical example is the previously described rectangular window of polycrystalline germanium.
For non-rectangular windows, e.g., circular windows, which are resistively heated, the equipotential lines are not linear. Thus, the electric field lines and, consequently, the paths of constant current, are elliptical. This results in a very non-uniform heating pattern because the power dissipation as given by potential field theory is related to the size of the equipotential/constant current squares.
Similar problems are encountered in optical distortions in high power laser system components. Present solutions include high pressure and high flow rates of a coolant past the optical components.